Oxides of nitrogen, sulfur dioxide, carbon monoxide, partly oxigenated hydrocarbons and other gases have become serious atmospheric pollutants. Significant atmospheric concentrations of such gases in large metropolitan and industrial areas from automobiles, industrial stack wastes, and consumption of certain fuels in homes and power generating facilities can cause serious pollution problems. Conditions such as smog are recognized as very detrimental to human and animal health, with the resulting eye and lung irritation being the most vexatious result. Such atmospheric contamination can also inhibit and destroy plant growth as well.
Sulfur dioxide, which has been found to be second only to carbon monoxide as a major pollutant source, is known to be extremely dangerous in view of its corrosive and potentially poisonous characteristics. It causes irritation and inflammation of the eyes and respiratory tract, and in moist air and fogs, combines with water to form sulfurous acid which is slowly oxidized into sulfuric acid.
The ability to continuously monitor the concentration of oxides of nitrogen, sulfur dioxide, carbon monoxide and other atmospheric pollutants has become increasingly important in remedying and controlling such conditions. Presently a number of pollutant concentration gas analyzers are available including those incorporating coulometric, colorimetric, electrical and thermal conductivity as well as infrared and ultraviolet method of analysis. More recently, electrochemical apparatus, such as those described in U.S. Pat. Nos. 3,622,487 and 3,622,488, have become available.
For the most part, these gas analyzers operate with a continuous supply of a representative gas sample taken from the environment under analysis. However, particulate matter, water and certain gases can interfere with the operation of the analyzer and must be removed, and the pressure and temperature of the sample must be regulated in many instances to avoid irregularities in the operation of or damage to the analyzer. In this regard, industrial stack gases, automobile exhausts and similar combustion gases present the most serious problems. Such combustion gases frequently contain soot, fly ash and water vapor which, if not removed from the sample, would quickly render most gas analyzer instruments inoperative. Moreover, the high temperatures and pressure fluctuations produced across any given cross-section of a stack, as well as the periodic variations at any point, can result in substantial inaccuracies for certain analyzers. In addition, if a gas sample containing water vapor is allowed to cool below its dew point, the condensed water vapor can remove significant amounts of such water soluble gases as SO.sub.2, NO.sub.2 and others, from the sample stream, thus reducing the concentration of those gases available to the analyzer.
Previous analyzers intended to operate within industrial stacks aare subject to interference or inaccuracy from one or more of these causes in spite of special sample conditioning systems. Attempts were made to supply "clean" gas samples to remote analyzers through banks of filters, chemical scrubbers and condensate removal apparatus that would eliminate particulate matter, water and interfering gases from the sample while cooling it to desired temperatures. However, such complex conditioning systems proved unduly cumbersome, fragile, and expensive and were not generally suitable for unattended operation in conjunction with a continuously operating analytical system. In addition, losses of certain water soluble gases are inherent in such system so that the concentrations in the samples reaching the analyzer could not reliably be correlated with those present in the stack or exhaust stream being monitored.
More recently, proposals have been made for use of a thin-wall transfer tube made of a heat resistant material permeable to the stack gases. With the tube extending across the stack transverse to the gas flow, clean dry air or other carrier gas is passed through the tube from one end to the other at a constant flow rate to carry the gas sample diffusing through the tube walls to an analyzer. In theory, diffusion through the transfer tube is supposed to eliminate the particulate matter and most of the water vapor from the diluted sample of the stack gas delivered to the analyzer. In practice, however, the permeation rate of the transfer tube varies with stack temperature so that a pyrometer control or the like must be used to compensate the analyzer operation for changes in stack temperature. Also, compensations must be made for variations in the stack gas flow rates and pressure, at least below certain minimum levels. Moreover, the capillary openings or other diffusion mechanisms in the transfer tube walls can become rather quickly clogged with soot and the like to reduce the rate at which the gas can enter the sample stream, and the periodic replacements of clogged tubes can be a cumbersome and time-consuming task. High stack temperatures can also distort the wall thickness of the transfer tubes, and thus its permeability, unless certain materials which can withstand the highest anticipated stack temperatures without collapse or distortion are employed.
This invention provides a simple and inexpensive apparatus and method, capable of use with a variety of gas analyzers, for acquiring and preparing representative gas samples without loss of desired components of the gas sample while at the same time eliminating various interfering substances and other sources of analyzer inaccuracies.